Hi- We are adding a master suite over an unheated garage totaling approximately 600 sq.ft. on a home constructed in 2001. I am trying to minimize drafts and taxing of the existing HVAC system (we live in Buffalo, NY) without breaking the budget on insulation. Since it is an addition and will have complex vaulted ceilings with dormers, SIP's are probably out. We are also probably limited to 2x4 walls since that was how the house was built initially (trying to avoid the addition looking different from existing); however, I could be convinced otherwise. To this end, right now we were thinking of the following: Floors- 2x10 joists with 1" spray foam and then fill with cellulose. Walls - 2x4 (maybe 2x6) with 1" spray foam, enchanced batt fiberglass, 1" XPS foam board (exterior of OSB sheathing) and house wrap. Ceiling - 2x10 rafters with enhanced batt fiberglass (I don't know if we can use spray foam here because of need to ventilate or foam board beneath the roofing) Windows - Pella/Anderson Low-E/Argon windows. Obviously, we would like to minimize drafts and maximize R-Value. Any suggested changes or improvements would be GREATLY appreciated. Thank you in advance!

Roamer
Add 2" XPS under the 2x10 floor joists to provide a thermal break. Foam and tape the joints. With the spray foam & cellulose it will be an excellent assembly. Ideally the same thing on the walls & roof - although I'd put the XPS on the exterior of the walls. Roof: Unvented, with 2" XPS (foamed & taped) to the inside, dense pak cellulose in the cavity with the spray foam. Sounds like a lot, but with a room over the garage, you typically have five exterior surfaces so there can be a lot of heat loss. (By the way, I've found XPS quite a bit cheaper at commercial roofing suppliers.)
Bob Irving
RH Irving Homebuilders